If it is your desktop application then you just need to create an XmlReader for that file and it will load your file into memory and will make that file readable to you. After that you just need to read/parse that file. Go through the following link for help (
http://www.dotnetperls.com/xmlreader[
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If you want to read an XML file using a web-based application, and you want the user to upload that file, then first you should provide the user the option to upload the file to the web-server. Once file is available, you can read it the same way, but in that case you should check for the the schema validation of that XML file.
Hope you have got the basic idea about what to do :)